#include <iostream>
using namespace std;

int main()
{
	int day;
	int sum = 0;
	int count = 0; 
	cin >> day;
	for(int i = 1;;i++){
		for(int j = 0;j<i;j++){
			count = count + 1;
			if(count > day){
				break;
			}
			sum = sum + i;
		}
		if(count > day){
			break;
		}
	}
	cout << sum << endl;
	return 0;
}
